Those familiar with the operation and maintenance of steam generators understand that, during system downtime, the tubes of the steam generator must be inspected, and defective tubes plugged or replaced. It is also known that steam generator tubes may be preventively plugged, in advance of leaks actually occurring therein, if an examination of the interior of the tube indicates the presence of high strain which, in turn, indicates incipient breakage. As the strain increases, the susceptibility to stress corrosion cracking increases It is therefore an object of the present invention to provide a more effective means of examining tube interiors to detect the presence or not of such strain which, in turn, will indicate the likelihood or not for tube breakage during reactivated use of the tube.
Distortion of the tube interior shape, commonly called "denting", is known to be associated with strain in the metal of which the tube is made. However, a mere presence of "denting" is not sufficient, in itself, to provide information for predicting the life of a tube having "denting". Therefore, more accurate determination of the amount of such strain by more accurate determination of relevant interior distortion of the tube, is a means by which predictions regarding tube cracking can be improved. Thus, it is intended by the present invention to provide an interior probe for tubes, such as steam generator tubes, for more accurately sensing tube distortion from which, in turn, the extent of strain in the tube can be more accurately calculated.
For example, especially in modern pressurized water reactor steam generators, "denting" occurs during operation by reason of an accumulation of corrosion products between the tube support plates and the tubes themselves. This distortion results in strain which, if high enough, makes the tube susceptible to stress corrosion cracking. In the past, it was believed that the larger the dent the greater was the strain, and eddy current inspection techniques have therefore been used to evaluate the size of these "dents". Such non-destructive examination of steam generator tubes has been developed from previously known eddy current techniques for detecting defects in the tubing, such as existing cracks and the like, and the interpretation of eddy current signals indicating the existence of denting is based on a comparison with known signals from standards Although denting can thus be quantified, eddy current measurements for this purpose have been found to be generally insufficient for the forecasting of tube life because, at best, they measure only the average tube diameter at any given location within the length of the tube. Accordingly, tube leaks cannot be accurately predicted using eddy current measurements.
Another known technique for measuring denting is that which utilizes an eight-fingered probe to concurrently measure the several inside radii of the tube at eight "finger" locations about its circumference as the probe is drawn through the tube, by sensing the extent of deflection of respective strain gauges mounted on the fingers, which data is then used as input to a computer to calculate strain. An example of such a probe is disclosed in U.S. Pat. No. 4,341,113. Such electromechanical gauging represents an improvement over eddy current inspection techniques, but is subject to significant error at high strain locations because the data is limited to eight radii. In this regard, it was found that increasing the data points to twelve or sixteen causes the probe to become unwieldy, without significant increase in accuracy.
Inspection and testing of steam generator tubes over a period of many years with the prior art probes described hereinbefore have shown that neither type of probe provides sufficient correlation between the data obtained thereby and the tube strain to be able to predict with reasonable accuracy of the life of a tube and hence, which tubes should be taken out of service. Therefore, as a matter of practice, some tubes were often taken out of service, thereby reducing the service life of a steam generator, when they need not be taken out of service, and in some cases, tubes were left in service when they should not have been because leaks subsequently developed.
Both of the aforementioned interior probing techniques are primarily designed to accurately detect locations of maximum denting within the tube which, as previously mentioned, were thought to correspond with the locations of maximum strain. However, I have found that maximum strain in a tube does not necessarily occur at a location of maxiumum denting but, rather, may occur at different locations depending upon the distorted shape or profile of the tube. That is, tube strain due to denting is more aptly composed of a circumferential membrane component and a bending component, considering the effect of axial strain to be negligible. I define the membrane component as the change in the circumference of the tube divided by its original circumference, and the bending component as the local change of length compared to the initial length at the specific location of the membrane component. Thus, circumferential or hoop strain can be determined on the basis of such deformed interior profile of the tube. It therefore becomes important to know the complete circumferential profile of the tube at locations of particular interest within the tube interior, and it is intended by the present invention to provide such capability.
Another problem encountered with said prior art probe is that although they may indicate a "dent", they do not necessarily indicate the location of a "dent" relative to the length of the tube. The characteristics of the "dents" at the tube supports are the most important characteristics to be considered in forecasting tube life. The tube supports are not necessarily located at points fixed distances apart or at specific locations from the tube ends. The prior art probes will frequently provide signals at dents which can be confused with signals indicating the presence of a tube support.
This possibility of confusion of signals also arises with the rotatable sensing member on the probe of the invention. Thus, while the rotatable sensing member on the probe of the invention provides much more information on the characteristics of a dent, e.g., shape as well as radially inward size, it does not clearly indicate the location of a tube support since a dent of such characteristics may appear other than at a support. However, a fixed position eddy current coil of proper sensitivity can provide signals which clearly distinguish between a "dent" and a tube support. Accordingly, a fixed position eddy current coil is included in the probe of the invention not to detect tube distortion, as in the prior art, but to provide signals which clearly indicate the locations of tube supports as distinguished from "dents".
An eddy current coil for indicating the location of the tube supports is also useful for controlling the speed at which the probe is drawn through a tube being inspected. In the prior art, it has been the practice to pull the probe through a tube at a relatively low speed in order to provide what was believed to be sufficient information for identifying "dents". However, since the "dents" of most importance are located at the tube supports much time is wasted, and the inspection of a tube is lengthy, during the travel of a probe between tube supports.
Accordingly, the invention provides not only the capability of measuring tube profile distortion in three dimensions, i.e. radially, axially and circumferentially of a tube, but also the capability for determining locations of exterior support plates along the length of the tube, which information can be correlated with the tube profile measurements to determine where along the tube length any such distortion has occurred and can be used to vary the speed of movement of the probe through a tube. Such knowledge of the precise locations of tube distortions affords a more positive understanding of the causes of tube distortions.